The invention relates to a contact adhesive and the method for producing the same which is to be applied to a suitable backing or adhesive carrier without the use of a solvent or dispersal medium. These adhesives are used on self-adhesive products, for example, on Kraft packaging tapes or other similar self-adhesive tapes and as a backing for sheets of self-sticking covering material and the like. The invention also relates to the self-adhesive product and the method for making the same.
The prior art methods of applying such an adhesive onto a suitable carrier required that the adhesive be dissolved or dispersed in a suitable organic solvent. The solvent was thereafter applied to the carrier or support material and evaporated, leaving behind the pressure sensitive contact adhesive compound. Unfortunately, this process results in a large amount of solvent vapors being released into the atmosphere with the accompanying health hazard to a person breathing these vapors and a possible fire or explosion hazard due to the inflammable nature of many solvents. As the carrier material is frequently made of a synthetic polymeric material, an explosion hazard compounded by the danger of an electrostatic charge building up and sparking, especially at high processing rates, providing an ignition source for the organic solvent vapors, is prevalent.
A further disadvantage of the prior art method is that the rate of production is relatively slow due to the method of application of the adhesive.
Attempts to avoid the use of solvents and to apply hot-melt adhesives to the backing, by working at higher temperatures, within the fusion range or melting point range of the adhesives have previously been less than successful. It has been found that the higher temperatures result in problems which manifest themselves in a number of ways and, depending on the final use of the material, need to be at least partially solved.
The most common problem involves the oxidation and temperature stability of the adhesive mixtures. Temperatures in excess of 100.degree. C are normally used for the mastication or homogenization process and, for the application of the adhesive to the backing, the adhesive must have a low enough viscosity to allow its application to the backing.
It has been a frequent practice to use material with a low value of polymerization and a large proportion of plasticizers, in order to prepare an adhesive with a low viscosity. Unfortunately, this results in a contact adhesive with a low endurance limit making them less suitable for jobs which require good adhesive strength while under continuous stress. The resulting contact adhesives could therefore only be used for certain purposes wherein continuous loads or stresses are not applied to the secured together parts.
Attempts to prepare a suitable hot-melt adhesive which can be applied to a suitable backing material or adhesive carrier, include the use of styrene and butadiene copolymers or styrene and isoprene copolymers with the addition of modified or unmodified rosin or rosin esters, coumarone-indene-resins, polyterpene-resins, aliphatic diene-olefin-hydrocarbon resins, polystyrene resins, copolymers of ethylene, vinyl acetate or polypropylene. Copolymers of vinyl esters with additions of various waxes have also been tried.
As noted above, however, these adhesives have been found inadequate in that their resistance to continuous load or stress is of too low an order of magnitude. A typical adhesive produced by the prior art methods, normally has a continuous load and stress resistance of only a few minutes when a load of 1.5 kg at 25.degree. C is applied to a tape having a width of 1 cm.
Hot-melt adhesives prepared using a base of ethylene-vinyl acetate copolymers and chlorinated hydrocarbons have found only limited use because of their low stress resistance. In particular, they are used in the laying of carpets in order to make the bottom side of the carpet "non-skidding".
Attempts to prepare a hot-melt adhesive using rubber as a base, have similarly been less than satisfactory. Thus, it has not been previously possible to prepare adhesives which can be applied to a suitable backing in a melt form, and which have or retain a high resistance to continuous loads and stresses. This type of material has great application in, for example, the packaging industry wherein packaging tapes capable of withstanding large continuous loads without releasing from the package, are in demand.